A few, one or two of which I've already mentioned.
1. The Tisbury area of Wiltshire. Extremely accessible by train, as most people on here can probably appreciate. Still unspoilt and deeply rural with a very nice landscape of rolling hills and a distinct lack of sprawling development. Tisbury itself is a large village/small town but is basically situated on rural country lanes which has presumably prevented expansion. A rare case of somewhere better served by train than by road, arguably. The complex geology with chalk hills two or three miles to the N and S together with greensand (I think?) hills nearby leads to a complex and interesting landscape.
2. The Teme Valley of Worcestershire. Haven't visited since 1998 but consists of a steep-sided valley and many smallish, steep-sided hills. I'd also add, in the same kind of area, the wooded hills immediately north of the Malvern Hills but lower, near the village of Knightwick on the A44. Basically I walked all the way from the Malvern Hills to the Church Stretton area, cross-country. The only problem I had, as I said in the other thread, is that the area was so little-frequented that many of the footpaths were in an appallingly bad state, being overgrown with trees and bushes and frequently blocked by barbed wire. Signposting often very poor too. Hopefully this has improved since as I believe a real effort to fix footpaths was made in the Blair years.
3. Not a million miles away from the above: the stretch of eastern Wales from Welshpool over to Bala. Again, visited some time ago (1999) and again I sampled it via a long-distance walk from the Church Stretton area eventually to the Snowdon area. Some similarity to the English counties the other side of the border: many steep-sided, green hills and quiet, unspoilt countryside. I distinctly remember the village of Pont Robert having a nice pub halfway though a day's walking through deserted countryside. This then leads into the Berwyn Mountains which are less craggy than other North Wales ranges but also less popular and quieter, which provides much appeal. There was a very nice ridge walk at the top.
The whole area did suffer from the same issues as Worcestershire though, namely blocked and poorly-signposted footpaths, so much so that at one point, I was very much behind schedule and had to give up and do the last 5 miles or so by road. Even that was scenic enough...
4. The Black Mountain (Mynydd Du) area in South Wales and the Llandovery area immediately to the north of it. Less frequented than the more eastern summits but a nice combination of high upland country and quiet rolling, green hills.
5. Not sure if the Vale of Rheidol counts, or whether the railway makes it too popular. But the footpath was quiet.